1. Field of the Invention
The present invention relates to an apparatus and method for reducing the flow of groundwater through a predetermined subsurface region. In particular, the present invention is a method and apparatus for minimizing the spread of subsurface contaminants by providing a temporary barrier to groundwater movement surrounding the contaminated region.
2. Discussion of Background
Contamination of subsurface soils and groundwater damages the local ecosystem, posing serious potential health problems if local groundwater is used as a source of drinking water or irrigation water, or if the soil is used for growing crops. Migrating groundwater may spread the contaminants to adjacent soils or water supplies. Groundwater contamination is among the most important pollution problems of the industrialized nations. As reliance on groundwater for drinking water, irrigation, and industrial uses increases, the problem will assume increasing urgency.
Contaminated soil can be treated by excavating the contaminated materials, including soils, sediments, fractured rock, and the like, The excavated materials are returned to the site after treatment or simply stored elsewhere. Groundwater may be removed along with excavated soil or be pumped to the surface of the earth for treatment. These direct methods of dealing with contaminated groundwater are time-consuming and expensive.
Other methods are available to treat contaminated sites in situ. For example, bioremediation techniques include use of aerobic bacteria--known to metabolize hydrocarbons--at the contaminated site. Nutrients, such as oxygen, can be injected into the vadose zone to stimulate bacterial growth (Payne, et al., U.S. Pat. No. 4,945,988). Alternatively, a bacterial culture may be supplied to the site together with air and nutrients, thereby facilitating metabolization of hydrocarbons in the soil (Hater, et al., U.S. Pat. No. 4,850,745). The effectiveness of these techniques depends on soil conditions, including existing nutrient availability, moisture content, temperature, and pH. While bioremediation techniques are effective for treating some contaminants, many months of treatment may be needed before contamination is reduced to acceptable levels.
Volatile contaminants may be stripped--drawn from the soil by application of negative pressure--as in the systems described by Koerner, et al. (U.S. Pat. No. 4,842,448), Bastian, et al. (U.S. Pat. No. 4,745,850), and Knopik (U.S. Pat. No. 4,183,407). A number of treatment systems rely on networks of injection and extraction wells for stripping of contaminants. A treatment fluid such as air, water, or steam is injected into the soil and is drawn towards the extraction well by a vacuum. The fluid passes through the contaminated area, volatilizing and removing contaminants from the area. See, e.g., Udell, et al. (U.S. Pat. No. 5,018,576) (steam injection); Payne (U.S. Pat. No. 4,890,673) (air injection).
A horizontal well system for treatment of contaminated groundwater is described in commonly assigned U.S. Pat. No. 4,832,122 issued to Corey, et al. An injection well is positioned below a contaminant plume; an extraction well system is positioned above the plume. A treatment fluid is pumped into the injection well and exits through a series of spaced, multiple apertures. The apertures are large and numerous enough to allow fluid to flow freely therethrough, but narrow enough to keep soil particles from blocking the flow. The volatile contaminants in the plume are carried by the fluid to the extraction well. The contaminant-laden fluid is drawn into the extraction well, and pumped upwards to the surface, where the contaminants are separated from the fluid.
Nonvolatile contaminants must be fixed in situ or removed by excavating soil or pumping the groundwater to the surface for treatment. As seen in FIG. 1, the subsurface structure under the earth's surface 10 includes an upper, unsaturated or vadose zone, indicated generally at A, and a lower, saturated zone, indicated generally at B. Zones A and B meet at water table 12. Plume 14 is situated partly in vadose zone A and partly in saturated zone B. Remediation apparatus 20 includes pump 22 and well 24. Pump 22 draws groundwater from plume 14 into well 24, pumping the groundwater to the surface for treatment. Contaminants in the groundwater may be removed in on-line treatment module 26, and the purified water returned to the subsurface.
As contaminated groundwater is withdrawn from plume 14, fresh groundwater is drawn into plume 14, as indicated by arrows 30 (FIG. 2). As contaminated groundwater is withdrawn from well 24, it is continually replaced by fresh groundwater which dilutes the remaining contaminated water in plume 14. Decontamination of plume 14 therefore requires larger quantities of groundwater to be removed and treated than would be necessary if plume 14 could be successfully isolated from local groundwater flows. If plume 14 is in a region of high groundwater flow, previously uncontaminated groundwater may carry contaminants from plume 14 to downstream soils or water supplies.
Migration of contaminants in subsurface soil and groundwater can be reduced by the use of low-permeability barriers or flow diversion systems. Plugging compositions including the trioxysilanes (Burkhardt, U.S. Pat. No. 4,645,382), sodium silicate, and the polyacrylamides (Gadelle, et al., U.S. Pat. No. 4,705,431) can be injected into the soil to form an impermeable barrier surrounding the contaminated area. Conduits of relatively low hydraulic resistance, located around the circumference of a contaminated area below the groundwater level, divert groundwater flow around the area (Laws, U.S. Pat. No. 4,288,174). Construction of these systems requires substantial expenses in both materials and labor. Furthermore, the systems are subject to degradation, with the attendant possibility of leakage of contaminants.
There is a need for an efficient, cost-effective method for containment of subsurface contaminants. The method should reduce the flow of groundwater into and through the area being treated, thereby halting or slowing the spread of the contaminants beyond the contaminated area.